Use disk for persistent cache

lib/jekyll/cache.rb

@@ -1,11 +1,13 @@
 # frozen_string_literal: true
 
+require "digest"
+require "fileutils"
+require "pstore"
+
 module Jekyll
   class Cache
     extend Forwardable
 
-    def_delegators :@cache, :[], :[]=, :clear, :delete, :key?
-
     # Get an existing named cache, or create a new one if none exists
     #
     # name - name of the cache
@@ -13,31 +15,113 @@ module Jekyll
     # Returns nothing.
     # rubocop:disable Style/ClassVars
     def initialize(name)
+      @@base_dir ||= File.expand_path(".jekyll-cache/Jekyll/Cache")
       @@caches ||= {}
-      @@caches[name] ||= {}
-      @cache = @@caches[name]
+      @cache = @@caches[name] ||= {}
+      @name = name
+      FileUtils.mkdir_p(path_to)
     end
 
     def self.clear
-      @@caches ||= {}
-      @@caches.clear
+      delete_cache_files
+      @@caches.each_value(&:clear)
     end
     # rubocop:enable Style/ClassVars
 
-    # rubocop:disable Lint/UselessSetterCall
-    def self.clear_if_config_changed(config)
-      cache = Jekyll::Cache.new "Jekyll::Cache"
-      clear unless cache["config"] == config
-      cache["config"] = config
+    def clear
+      delete_cache_files
+      @cache.clear
     end
-    # rubocop:enable Lint/UselessSetterCall
 
-    def getset(key)
-      if key?(key)
-        @cache[key]
+    def [](key)
+      return @cache[key] if @cache.key?(key)
+      path = path_to(hash(key))
+      if File.file?(path) && File.readable?(path)
+        @cache[key] = load(path)
       else
-        @cache[key] = yield
+        raise
       end
     end
+
+    def getset(key)
+      return @cache[key] if @cache.key?(key)
+      path = path_to(hash(key))
+      if File.file?(path) && File.readable?(path)
+        value = load(path)
+      else
+        value = yield
+        dump(path, value)
+      end
+      @cache[key] = value
+    end
+
+    def []=(key, value)
+      @cache[key] = value
+      path = path_to(hash(key))
+      dump(path, value)
+    end
+
+    def delete(key)
+      @cache.delete(key)
+      path = path_to(hash(key))
+      File.delete(path)
+    end
+
+    def key?(key)
+      return true if @cache.key?(key)
+      path = path_to(hash(key))
+      File.file?(path) && File.readable?(path)
+    end
+
+    # rubocop:disable Style/ClassVars
+    def self.clear_if_config_changed(config)
+      config = config.inspect
+      cache = Jekyll::Cache.new "Jekyll::Cache"
+      unless cache.key?("config") && cache["config"] == config
+        delete_cache_files
+        @@caches = {}
+        cache = Jekyll::Cache.new "Jekyll::Cache"
+        cache["config"] = config
+      end
+    end
+    # rubocop:enable Style/ClassVars
+
+    private
+
+    def path_to(hash = nil)
+      @base_dir ||= File.join(@@base_dir, @name)
+      return @base_dir if hash.nil?
+      File.join(@base_dir, hash[0..1], hash[2..-1]).freeze
+    end
+
+    def hash(key)
+      Digest::SHA2.hexdigest(key).freeze
+    end
+
+    def delete_cache_files
+      FileUtils.rm_rf(path_to)
+    end
+
+    # rubocop:disable Security/MarshalLoad
+    def load(path)
+      cached_file = File.open(path, "rb")
+      value = Marshal.load(cached_file)
+      cached_file.close
+      value
+    end
+    # rubocop:enable Security/MarshalLoad
+
+    def dump(path, value)
+      dir, _file = File.split(path)
+      FileUtils.mkdir_p(dir)
+      cached_file = File.open(path, "wb")
+      Marshal.dump(value, cached_file)
+      cached_file.close
+    end
+
+    def self.delete_cache_files
+      FileUtils.rm_rf(@@base_dir)
+    end
+    private_class_method :delete_cache_files
   end
 end